1. Field of the Invention
The present invention relates to a semiconductor laser diode and, more particularly, to a semiconductor laser diode with high optical power capabilities and reliability.
2. Discussion of the Related Art
With the ever-growing importance of III-V compound semiconductor laser diodes for applications such as optical communications, printers and optical disk memory systems, there is a great demand for high reliability device providing for long lifetimes coupled with high power capabilities.
For these devices, the maximum optical output power is generally limited by catastrophic optical damage (COD) occurring due to local heating at the laser mirrors. It is thus important that the COD level can be raised by optimizing the critical mirror facet region with respect to a number of factors determining device performance such as the absorption of optical energy, the number of electric carriers and their recombination rate at the facet as well as the mirror passivation, to reduce heat development near the mirror interface.
Such optimization is of importance for the hitherto most commonly used laser diodes having cleaved mirrors but also, and even more so, for etched mirror devices to which, more recently, much attention has been directed. This mainly because of the inherent advantages of this technology which allows full wafer processing and testing and a high level of integration. A typical device and its fabrication is disclosed in European patent application 0 363 547 "Method for Etching Mirror Facets of III-V Semiconductor Structures". However, the processes required to etch, clean and passivate the etched facets tend to have a harmful effect on the mirror quality by increasing the number of surface states whereby it becomes very difficult to achieve the required performance characteristics. There is a definite need for improved high reliability high-power devices.
A non-absorbing mirror (NAM) device can be realized with a bent-waveguide structure where the optical beam is emitted through a window structure of wider bandgap material, which may be a section of a cladding layer. A rather large variety of devices incorporating this concept has already been proposed, e.g., in the following documents:
Patent Abstracts of Japan, Vol. 9, No. 105 (E-327)(1988), Jul. 10, 1985. "Semiconductor Laser Element". PA1 EP-A-0 332 723 "High-Power Semiconductor Diode Laser". PA1 DE-A-3 604 293 "Heterostruktur-Halbleiterlaserdiode". PA1 EP-A-0 069 563 "Semiconductor Laser Diode". PA1 Article "Lateral p-n Junction Formation in GaAs Molecular Beam Epitaxy by Crystal Plane Dependent Doping" by D. L. Miller (Appl. Phys. Lett., Vol. 47, No. 12, December 1985, pp. 1309-1311). - EP-A-0 261 262 "Transverse Junction Stripe Laser". PA1 Article: "High-power Fundamental Mode AlGaAs Quantum Well Channeled Substrate Laser Grown by Molecular Beam Epitaxy" by H. Jaeckel, et al. (Appl. Phys. Lett. 55(11), Sep. 11, 1989, pp. 1059-1061).
In each of the devices described in these documents, the bent-waveguide structure is obtained by growing the layered diode structure on a patterned substrate surface having a long horizontal mesa center region on which the active gain section of the waveguide is grown and, near its ends where the facets are later on to be cleaved or etched, short inclined regions or edges perpendicular to the longitudinal direction of the waveguide. The angle between horizontal and inclined surfaces is large enough to force the beam generated in the active waveguide section not to follow the bent waveguide but to proceed essentially undeflected through the (wider bandgap) surrounding material to the mirror facet.
With such NAM structures substantial performance improvements have been achieved in that the optical absorption at the facet can be largely reduced. However, the required long-time reliability at high-power output still has not been reached when relying on a NAM-structure alone. For etched mirror devices in particular, additional measures need to be taken to satisfactorily solve the heating problem.
For certain material systems such as GaAs/AlGaAs, the conductivity-type, p or n, of epitaxially grown layers depends on the crystallographic orientation of the substrate surface when an amphoteric dopant such as Si is being used. On structured surfaces having horizontal planar mesas and inclined adjacent regions, the material grown on planar regions will be of a first conductivity-type whereas the material above the inclined regions will be of the second conductivity-type, with p-n or n-p junctions formed in between. Devices can be designed to use these junctions to either allow or to prevent currents from flowing in the lateral direction.
Again, a variety of documents are known that describe the crystal-plane-dependent doping effect and some applications thereof. The following are believed to be representative of the state of the art:
In each of these publications, the described structures are deposited on a patterned substrate surface to obtain doping reversal in the semiconductor grown on inclined surface regions that are normally very short, forming an edge that runs parallel to the longitudinal direction of the waveguide eventually formed. This is in contrast to the device structures and substrate pattern disclosed in the earlier cited publications on NAM-structures, where the inclined surface regions or edges extend, in all instances, in a direction perpendicular to the active waveguide. Edges parallel to the waveguide are appropriate for the purposes for which the p-n junctions are being used: they serve to provide lateral current confinement over the length of the waveguide; they are not employed to prevent currents from flowing into the critical mirror facet regions of the laser device.
It would thus be desirable to provide a laser diode device that provides for reduction of optical power absorption, the elimination of lateral current flow towards the mirror facet region of the laser diode, and improved reliability.